This disclosure concerns two novel electrically conducting organic oligomers: oligo(3-amino-1H-pyrazole-4-carbonitrile) or “oligo(AP-CN)” and oligo(4-nitro-1H-pyrazole-3-yl-amine) or “oligo(AP-NO2)”.
These novel electrically conducting organic oligomers have highly variable redox states and good electron-transporting properties. Our studies also show that the oligomers may be useful in applications such as polymer solar cells.
These oligomers are easy to synthesize, requiring only one step plus purification. These oligomers use inexpensive starting materials.
In photocurrent generation studies using a solar lamp and an electrolyte with a sacrificial electron donor, the oligo(AP-CN) was able to produce anodic photocurrent of magnitudes as high as 103 times that of a gold-coated electrode alone, and 43.2 times that of a fullerene-coated gold electrode. Chemical characterization of oligo(AP-CN) showed that it is a tetramer with N-linkages between repeat units. It has a high thermal stability, with an onset of thermal decomposition above 350° C.
Cyclic voltammetry and electrochemical impedance spectroscopy (EIS) studies of both oligomers showed that they are good electron conductors when in the reduced (n-doped) state. The n-dopability is quasi-reversible. This observation is further supported by circuit models that give low values for the resistor and Warburg circuit elements in when n-doped.
When partially or mostly oxidized (at 0 V or +1.2 V), the oligomers may act as p-doped conductors and semiconductors. The high electron conductivity quantified by the EIS is consistent with the observations of the large anodic (electron) photocurrents supported by the oligo(AP-CN).
Conducting polymer-based photovoltaic cells are likely to be a much less expensive alternative for solar energy than traditional inorganic photovoltaics made from such materials as silicon and gallium arsenide. Inorganics require high temperature and high vacuum processing conditions, such as molecular beam epitaxy. Up to 40% of the cost of a silicon photovoltaic cell arises from the material processing. Several types of conducting polymers, however, have been made to be liquid processable at room temperature using inexpensive methods. They can be deposited on large sheets using ink-jet printing, screen-printing, or spin-casting. As thin films they are also mechanically flexible, able to withstand bending that would fracture a silicon panel. They are also color tunable within various ranges, so that they can be made to emit or absorb in a variety of colors.
The challenges within conducting polymer photovoltaics are low photon-to-current conversion efficiencies and short lifetimes. The current record power efficiency for a polymer photovoltaic is 3.5%, which is a full order of magnitude lower than the record power efficiency for silicon photovoltaics. However, silicon photovoltaics have benefited from about 20 years of dedicated research, whereas polymer photovoltaics are a relatively new application.
To date, the vast majority of conducting polymers are p-dopable, that is, they act as stable carriers of positive charge. In a photovoltaic cell they are often used as hole (cation or cation-radical) transporters.
Conducting polymers that are stable in their n-doped state are far less common. In this state they are able to accommodate and conduct free electrons. Such polymers are of significant value in photovoltaics as electron transporters, as well as in other applications such as organic field effect transistors (OFETs) and organic light emitting diodes (OLEDs). A common material used as the electron transporter in polymer solar cells is C60 fullerene, either pristine or derivatized in various manners to affect electrode morphology.
Some of the few examples of n-dopable conducting and semi-conducting polymers are based on nitrogen-rich 5-membered conjugated heterocycles such as 1, 3, 4 heterodiazoles containing C, N, and S or O. A few others are based on 1, 2, 4 triazoles having two C and three N atoms. The latter are n-dopable because the ring is electron deficient and can thus be reduced into a semi-stable, electron conducting form.
The electron deficiency also results in another phenomenon—the synthesis of homopolymeric poly(triazoles) is not generally achievable by polymerization of monomeric triazole. Thus, other approaches must be used. For example, one may construct linear polyhydrazides having —CR—CR—NH—NH— repeating sequences, and then undertake ring-formation metathesis reactions that yield the triazole repeat unit.
The synthesis of these types of prior art polymers usually requires many steps, often involving the complex ring-closing reactions mentioned above.
Another, much less explored approach to the synthesis of n-dopable conducting polymers involves oxidative polymerization or oligomerization of nitrogen-rich pyrazoles with primary amines. This is done in a manner similar to the synthesis of polyaniline, wherein the amine group forms a bridge between each polymer repeat unit. One example, oligo(3-amino-1,2,4 triazole), has been synthesized and characterized as a semiconductor and as a anticorrosive for copper. This material was not n-dopable to a stable state, however, and is not an effective electron acceptor.
These oligomers disclosed herein are expected to have a significant military and commercial interest. There is a necessity for developing renewable energy sources given that world petroleum production is expected to peak within the next few years. Solar power is clean, readily available, and renewable. Silicon-based solar cells are considered to be the state of the art at present and have a relatively high efficiency, but they are expensive to manufacture.
Polymer-based solar cells are less expensive but the prior art efficiencies are low. When incorporated into these types of cells, the oligomers herein disclosed will enable increases in efficiency. This is because of the high electron transport rates of the oligomers and their versatile redox behavior.
The ease of synthesis of these currently disclosed oligomers and low cost of starting materials are further commercial advantages. These attributes also allow derivatives of the material to be produced and investigated easily and quickly.